Power supply systems designed to supply electrical energy to low impedance loads need to address a number of specific problems. When standard power supply transformer technology is used it is difficult to limit the ultimate output current delivered to a low, impedance load. Potentially high output currents can be generated using standard transformer technology for a low impedance load which can result in damage to the components of the power supply system and/or the load which is to be supplied with electrical energy.
One approach used to restrict the output current supplied to low impedance loads is to place a resistance in line with the load. The resistance used is selected to keep the output current of the transformer at manageable levels for the voltage required by the load. However, one problem associated with this resistant based approach is the amount of waste heat generated by the resistor which needs to be dissipated by the power supply system. To dissipate heat a power supply generally needs to incorporate a fan or other similar cooling components. Including these components can increase the size, complexity and overall cost of the power supply provided. Furthermore, where such power supplies are to be used in dusty or chemically corrosive environments, air driven by a cooling system through the housing of a power supply can over time damage the components of the supply.
The use of resistive elements to control transformer output current also degrades the power transfer efficiencies of the supply. In general terms, resistors deployed in line or in series with a load will not match the impedance of the load with that of the supply, thereby limiting the efficiency of power transfers completed through to the load.
Previous attempts at providing a power supply system designed to supply electrical energy to low impedance loads have been made. For example, U.S. Pat. No. 2,992,386 discloses a way of compensating for variations in the input voltage of a transformer, so that the output voltage of the transformer remains stable. This invention works by having a section of the transformer core which is “saturable” or non-linear. On this section is wound a coil, to which is connected a capacitor. The coil and capacitor combination is designed so that at the minimum operating voltage of the transformer, the coil/capacitor combination start to saturate the core. As the input voltage increases, the saturation of the core also increases, resulting in a change of the path taken by the magnetic flux of the transformer. The different flux path compensates for the increased input voltage.
One of the embodiments of the invention shown in U.S. Pat. No. 2,992,386 discloses the use of two separate transformers, one saturable and the other one wound as an auto-transformer. Whilst it is mentioned that a toroid could be used as the auto-transformer, there is no mention that a toroid could be used in conjunction with a “shunt” (or used as a saturable core).
U.S. Pat. No. 2,992,386 does not mention current limiting at all. Rather, the word “shunt” in this prior invention is used to describe an alternative magnetic path, which is used to provide voltage regulation. In this way, the operating principle of this prior invention relies on the effect of ferro-resonance. Furthermore, whilst a “toroid” is mentioned in the patent, it is mentioned in the context of a convenient way to incorporate an auto-transformer winding.
U.S. Pat. No. 4,422,015 discloses an invention to limit the current for an insect trap, which utilises magnetic shunts to introduce current limiting flux leakage. Accordingly, as the invention relates to an insect trap, the invention operates at high frequency (the circuits cited in this patent operate at frequencies of at least 30 Khz and do not produce large currents. Furthermore, the invention disclosed in U.S. Pat. No. 4,422,015 does not disclose the use of a toroid to introduce current limiting flux leakage.
U.S. Pat. No. 3,387,203 discloses a transformer arrangement which is a modification to a particular frequency generator design, which was typically used as a ring generator in telephone exchanges. In other words, the invention disclosed in U.S. Pat. No. 3,387,203 is not intended as a power supply.
The invention discloses a toroidal transformer which has been modified to eliminate an inductor from a prior-art frequency generator design. This is achieved by the separation of the transformer windings and the addition of a magnetic shunt. The resulting toroid and shunt arrangement was an upgrade to a prior-art frequency generator (see FIG. 2).
In order to work, the toroid and shunt arrangement need to be carefully designed and manufactured so as to be part of a tuned circuit. This invention requires precise air gaps between the shunts and the transformer core. The toroid core and the shunts are made from specific materials, in order to operate at the correct frequency and with the correct losses.
It would be of advantage to have an improved power supply and/or improvements available to existing power supplies which mitigated the above problems. In particular, an improved power supply capable of managing output currents while minimising the generation of waste heat would be of advantage. A power supply system which could also effectively match the impedance characteristics of the supply with the impedance of a particular load for efficient power transfers would also be of advantage.